1. Technical Field
The present invention relates generally to driving a signal on a wireline and more particularly to feedback impedance control for driving a signal.
2. Description of Related Art
A line driver drives a signal that is suitable for transmission over a wired connection. The difficulty of driving a signal onto a wired connection increases when the impedance on the wired connection is unknown and dynamic. One example of wired connections is a powerline network used for communications such as a residential powerline.
One limitation with powerline is that the impedance of the network can be unknown. There are regulations and standards that specify emission and performance requirements for a device connected to the powerline for a known impedance such as 50 ohms. However, the actual impedance of powerline can have a wide range such as from 5 ohms to 300 ohms, both from one node to another node and within the frequency range utilized in the communications system. The impedance has a wide range because the impedance depends on the network topologies (e.g. star and ring), the wiring, the connections, and the devices or appliances plugged into the sockets of the powerline with the effects of reflections combining with the passive characteristics to make a complex impedance. Furthermore, the impedance of the powerline can be dynamic (i.e. change during operation). For example, a timer may turn on a heater, which significantly increases the load upon the powerline.
Another limitation is that the impedance changes with frequency. In a band of 2 MHz to 30 MHz, the impedance is different at 2 MHz than at 30 MHz. Plus, the impedances at the different frequencies behave like a complex load as opposed to a simple load such as a resistor.
Some wired communications systems use complex multi-carrier modulation schemes that put particular requirements on inter-carrier modulation interference to achieve performance or even pass regulations. In powerline, high density orthogonal frequency division multiplexing (OFDM) typically uses a large number of carriers such as greater than 200. The non-linearity in the line driver may cause interference, which can degrade the performance of the wireline (e.g. degrading signal-to-noise ratio in other carriers). This non-linearity in the line driver may cause problems for other modulation schemes.
A line driver can use two different basic drive schemes: voltage buffering and current steering, voltage buffering being the more common. The challenge with voltage buffering is to prevent distortion by trying to drive too much current for the design when the impedance is low. The challenge with current steering is to prevent distortion by generating too much voltage than was designed for when resistance is high.